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    封装一个函数 sendAJAX 发送 GET AJAX请求
    参数 URL
    返回结果 promise对象
    <script>
        function sendAJAX(url){
            return new Promise((resolve, reject) => {
                const xhr = new XMLHttpRequest()
                xhr.responseType = 'json'
                xhr.open('GET',url)
                xhr.send()
                // 处理结果
                xhr.onreadystatechange = function(){
                    if(xhr.readyState === 4){
                        // 成功
                    if(xhr.status >= 200 && xhr.status < 300){
                        resolve(xhr.response)
                        // 失败
                    }else{
                        reject(xhr.status);
                    }   
                }
                }
            })
        }
        sendAJAX('http://ajax-api.itheima.net/api/news')
        .then(value => {
            console.log(value)
        },reason => {
            console.warn(reason)
        }
        )
    </script>
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